[WORKFLOW SDK FEATURE REQUEST] Use `grpc.aio` in the durabletask worker

`TaskHubGrpcWorker` consumes the sidecar stream on a dedicated `StreamReader` thread and pushes things to the event loop through a queue. We already have grpc.aio in the SDK, we can consume it asynchronously and remove the thread and the queue.

This should not change the public API.

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Acceptance criteria:

- [ ] The worker consumes the stream asynchronously
- [ ] `WorkflowRuntime.start()` / `.shutdown()` still works
- [ ] Reconnect on stream drop and on transient errors still works
- [ ] No leaked resources on shutdown